This invention relates to an antenna device comprising a scanning reflector and a fixed feeder interacting with the reflector for emitting radar radiation.
In such an antenna device the scanning position, or scanning angle, needs to be accurately determined in order to be able to determine the lateral position of detected objects. For example, when provided in a car radar apparatus it is necessary to determine in what lane a detected obstacle is.
A known device, that could be used in the above antenna device, for detecting the scanning position at a high accuracy is a resolver sensor. A resolver sensor comprises two coils positioned at right angles in a magnetic field. The distribution of the magnetic field on the two coils is detected and used as a measure of the scanning position. However, the construction of this resolver sensor is complex rendering the sensor expensive. This is particularly a drawback when applied in a car radar apparatus.
An object of the present invention is to provide an antenna device having a simple low cost yet accurate determination of the scanning position of the reflector; and a method for performing said determination.
The object is achieved by an antenna device and a method according to the enclosed claims.
In one aspect thereof, the invention relates to an antenna device comprising a scanning reflector, a fixed feeder interacting with the reflector for emitting radar radiation and a detector for detecting reflector passage of at least two different predetermined scanning positions during scanning of the reflector. The antenna device further comprises timing means for determining passage times at said passages, and prediction means for predicting the scanning position of the reflector at an arbitrary time on basis of said passage times and a predetermined scanning motion of said reflector.
In another aspect thereof, the invention relates to a method for determining an scanning position for the scanning reflector of the above described antenna device. The method comprises the steps of:
measuring passage times for the reflector, during reflector scanning, passing at least two predetermined and spaced scanning positions;
predicting the scanning position of the reflector at an arbitrary time by means of said passage times and a predetermined scanning motion of said reflector.
In accordance with the present invention, by arranging for the measurement of passage times, at two or more spaced predetermined scanning positions, accurate relationships between position and time are established. The scanning reflector of the antenna is operated in a predetermined way as regards the motion thereof. Consequently, the momentary position of the reflector is theoretically known. However, due to mechanical and other deviations a degree of inaccuracy exists in practice. The invention substantially reduces that inaccuracy. By means of said theoretically known movement and the actual and precise time measurements at the predetermined, and accurately known, passage positions accurate predictions of scanning position at arbitrary times are enabled. By the expression arbitrary times is to be understood other points of time than the measured passage times.
Further objects and advantages of the present invention will be discussed below by means of exemplary embodiments.